Glutathione (GSH), a tripeptide, is the major non-protein thiol in eucaryotic cells. The high level of GSH in mammalian and avian testis and its sharp rise during weeks 2-4 of postnatal life in the rat have suggested important functions for this compound during spermatogenesis. Doubtless, these include the processing of cysteine containing proteins and the concentration of protein-bound Se by testis and sperm. However, the early timing of the postnatal increase in GSH suggests that it may be necessary for events preceding spermiogenesis. In particular, there is considerable circumstantial evidence that GSH functions critically during meiosis. Aside from the timing of its increase in rat testis, this includes: (1) the ability of known or probable substrates for GSH S-transferases to stop spermatocyte development, and (2) the known protective effect of thiols against radiation-induced chromosome aberrations in testis and various cell culture systems. It is proposed to investigate the biochemical mechanisms responsible for the postnatal GSH increase in rat testis and to obtain further information concerning its relevance to meiosis, protection of spermatogenesis against harmful drugs and radiation, seasonal cycling, and processing of cysteine-rich and Se-containing proteins. During these studies, it is planned to gain additional perspective on the functions of testis GSH by comparative studies in lower vertebrates, in some of which (e.g. fish and frogs) the concentration of GSH appears to be much less than in mammals.